The invention relates to a cathode ray tube provided with at least one oxide cathode comprising a cathode carrier with a cathode base of a cathode metal and a cathode coating of an electron-emitting material containing an alkaline earth oxide selected from the group formed by the oxides of calcium, strontium and barium and a rare earth metal.
A cathode ray tube is composed of 4 functional groups:
electron beam generation in the electron gun,
beam focusing using electrical or magnetic lenses,
beam deflection to generate a raster, and
luminescent screen or display screen.
The functional group relating to electron beam generation comprises an electron-emitting cathode, which generates the electron current in the cathode ray tube and which is enclosed by a control grid, for example a Wehnelt cylinder having an apertured diaphragm on the front side.
An electron-emitting cathode for a cathode ray tube generally is a punctiform, heatable oxide cathode with an electron-emitting, oxide-containing cathode coating. If the oxide cathode is heated, electrons are evaporated from the electron-emitting coating into the surrounding vacuum. If the Wehnelt cylinder is biased with respect to the cathode, then the quantity of emergent electrons and hence the beam current of the cathode ray tube can be controlled.
The quantity of electrons that can be emitted by the cathode coating depends on the work function of the electron-emitting material. Nickel, which is customarily used for the cathode base, has itself a comparatively high work function. For this reason, the metal of the cathode base is customarily coated with another material, which mainly serves to improve the electron-emitting properties of the cathode base. A characteristic feature of the electron-emitting coating materials of oxide cathodes in cathode ray tubes is that they comprise an alkaline earth metal in the form of the alkaline earth metal oxide.
To manufacture an oxide cathode, a suitably shaped sheet of a nickel alloy is coated, for example, with the carbonates of the alkaline earth metals in a binder preparation. During evacuating and baking out the cathode ray tube, the carbonates are converted to the oxides at temperatures of approximately 1000xc2x0 C. After this burn-off of the cathode, said cathode already supplies a noticeable emission current which, however, is still unstable. Next, an activation process is carried out. This activation process causes the originally non-conducting ionic lattice of the alkaline earth oxides to be converted to an electronic semiconductor in that donor-type impurities are incorporated in the crystal lattice of the oxides. These impurities essentially consist of elementary alkaline earth metal, for example calcium, strontium or barium. The electron emission of the oxide cathodes is based on the impurity mechanism. Said activation process serves to provide a sufficiently large quantity of excess, elementary alkaline earth metal, which enables the oxides in the electron-emitting coating to supply the maximum emission current at a prescribed heating capacity. A substantial contribution to the activation process is made by the reduction of barium oxide to elementary barium by alloy constituents (xe2x80x9cactivatorsxe2x80x9d) of the nickel from the cathode base.
For the function and the service life of an oxide cathode it is important that elementary alkaline earth metal is continuously dispensed. The reason for this being that the cathode coating continuously loses alkaline earth metal during the service life of the cathode. The cathode material partly evaporates slowly as a result of the high temperature at the cathode and is partly sputtered off by the ion current in the cathode ray tube.
However, initially the elementary alkaline earth metal is continuously dispensed by reduction of the alkaline earth oxide at the cathode metal or activator metal. Said dispensation stops, however, when a thin, yet high-impedance interface of alkaline earth silicate or alkaline earth aluminate forms between the cathode base and the emitting oxide in the course of time. The service life is also influenced by the fact that the amount of activator metal in the nickel alloy of the cathode base becomes depleted in the course of time.
EP 0 482 704 A discloses an oxide cathode, the carrier of which is essentially composed of nickel and coated with a layer of an electron-emitting material comprising alkaline earth metal oxides, barium and a rare earth metal. The number of rare earth metal atoms in said electron-emitting material relative to the number of alkaline earth metal atoms being 10 to 500 ppm, said rare earth metal atoms being substantially uniformly distributed over the upper part of the layer of an electron-emitting material.
It is an object of the invention to provide a cathode ray tube, the beam current of which is uniform and remains constant for a long period of time, while said cathode ray tube can be reproducibly manufactured.
In accordance with the invention, this object is achieved by a cathode ray tube provided with at least one oxide cathode comprising a cathode carrier with a cathode base of a cathode metal and a cathode coating of an electron-emitting material with oxide particles, said oxide particles containing an alkaline earth oxide, selected from the group formed by the oxides of calcium, strontium and barium, doped with an oxide doping in a quantity ranging from 120 to maximally 500ppm of an oxide selected from the oxides of scandium, yttrium, lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium, and the electron-emitting material has an electric conductance of 3*10xe2x88x923 xcexa9xe2x88x921 cmxe2x88x921 to 12.5*10xe2x88x923 xcexa9xe2x88x921cmxe2x88x921.
The invention is based on the principle that, in a cathode ray tube comprising an oxide cathode, the service life of said oxide cathode is extended if the electrical conductance of the cathode coating is adapted to the operating point of the average direct current load on the cathode.
A cathode ray tube comprising such an oxide cathode has a uniform beam current for a long period of time because said controlled conductance of the cathode coating enables both excessive heating and cooling of the oxide cathode during operation of the cathode ray tube to be precluded. By virtue thereof, the operating temperature in the oxide cathode is optimal. As a result, also the reaction rate at which elementary barium is formed is optimal.
As barium is dispensed continuously, depletion of the electron emission, as known from the oxide cathodes according to the prior art, is precluded. Substantially higher beam current densities can be obtained without adversely affecting the service life of the cathode. This can also be used to draw the necessary electron beam currents from smaller cathode regions. The spot size of the hot spot determines the beam focusing quality on the display screen. The picture definition is increased throughout the screen. As, in addition, the cathodes age more slowly, picture brightness and picture definition can be maintained at a high level throughout the service life of the tube. Both the resolution and the brightness of the CRT are improved or, alternatively, when the brightness and the resolution remain unchanged, the operating temperature of the cathode can be reduced.
Within the scope of the invention it is preferred that the quantity of oxide doped is 240 ppm.
In comparison with the prior art, particularly advantageous effects are obtained by the invention if the oxide doping consists of Y2O3. As a result, the barium emission becomes more uniform both locally and in time. Oxide cathodes are obtained having a higher direct current loading capability and a longer service life.
It may be preferred that the oxide doping contains a sesquioxide selected from the sesquioxides of lanthanum, neodymium, samarium, cerium, praseodymium, gadolinium, terbium, dysprosium and holmium.
It is particularly preferred that the oxide doping comprises a sesquioxide selected from the sesquioxides of lanthanum, cerium, praseodymium and neodymium. In this type of cathode, the emphasis is placed on the insensitivity to poisoning, particularly oxygen poisoning. This type of cathode exhibits a uniform emission and can be reproducibly manufactured.
The invention also relates to an oxide cathode comprising a cathode carrier with a cathode base of a cathode metal and a cathode coating of an electron-emitting material with oxide particles, said oxide particles containing an alkaline earth oxide, selected from the group formed by the oxides of calcium, strontium and barium, doped with an oxide doping in a quantity from 120 to maximally 500 ppm of an oxide selected from the oxides of scandium, yttrium, lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium, and the electron-emitting material has an electrical conductance of 3*10xe2x88x923xcexa9xe2x88x921 cmxe2x88x921 to 12.5*10xe2x88x923xcexa9xe2x88x921 cmxe2x88x921.
These and other aspects of the invention will be apparent from and elucidated with reference to an embodiment described hereinafter.